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use std::sync::{
Arc,
atomic::{AtomicBool, Ordering},
};
use crossbeam::queue::ArrayQueue;
use super::{CellValue, Watchable};
use crate::{
cell::{Cell, CellImmutable, CellMutable},
signal::Signal,
};
pub trait BackpressureExt<T>: Watchable<T> {
/// Buffer values with drop-oldest strategy when capacity is reached.
///
/// When the buffer is full, the oldest value is discarded to make room.
/// Uses lock-free `ArrayQueue` internally.
///
/// # Example
///
/// ```
/// use hyphae::{Cell, Mutable, Gettable, BackpressureExt};
///
/// let source = Cell::new(0);
/// let buffered = source.drop_oldest(3);
///
/// // Fast producer, slow consumer scenario
/// source.set(1);
/// source.set(2);
/// source.set(3);
/// source.set(4); // Drops 1, keeps 2,3,4
/// ```
#[track_caller]
fn drop_oldest(&self, capacity: usize) -> Cell<T, CellImmutable>
where
T: CellValue,
Self: Clone + Send + Sync + 'static,
{
assert!(capacity > 0, "capacity must be positive");
let derived = Cell::<T, CellMutable>::new(self.get());
let weak = derived.downgrade();
let buffer: Arc<ArrayQueue<T>> = Arc::new(ArrayQueue::new(capacity));
let first = Arc::new(AtomicBool::new(true));
let guard = self.subscribe(move |signal| {
if let Some(d) = weak.upgrade() {
match signal {
Signal::Value(value) => {
if first.swap(false, Ordering::SeqCst) {
return;
}
// Try to push, if full drop oldest and retry
let val = (**value).clone();
if buffer.push(val.clone()).is_err() {
// Buffer full - drop oldest
let _ = buffer.pop();
let _ = buffer.push(val.clone());
}
d.notify(Signal::value(val));
}
Signal::Complete => d.notify(Signal::Complete),
Signal::Error(e) => d.notify(Signal::Error(e.clone())),
}
}
});
derived.own(guard);
derived.lock()
}
/// Buffer values with drop-newest strategy when capacity is reached.
///
/// When the buffer is full, new values are discarded (not buffered).
/// Uses lock-free `ArrayQueue` internally.
///
/// # Example
///
/// ```
/// use hyphae::{Cell, Mutable, Gettable, BackpressureExt};
///
/// let source = Cell::new(0);
/// let buffered = source.drop_newest(3);
///
/// // Fast producer, slow consumer scenario
/// source.set(1);
/// source.set(2);
/// source.set(3);
/// source.set(4); // Dropped, buffer has 1,2,3
/// ```
#[track_caller]
fn drop_newest(&self, capacity: usize) -> Cell<T, CellImmutable>
where
T: CellValue,
Self: Clone + Send + Sync + 'static,
{
assert!(capacity > 0, "capacity must be positive");
let derived = Cell::<T, CellMutable>::new(self.get());
let weak = derived.downgrade();
let buffer: Arc<ArrayQueue<T>> = Arc::new(ArrayQueue::new(capacity));
let first = Arc::new(AtomicBool::new(true));
let guard = self.subscribe(move |signal| {
if let Some(d) = weak.upgrade() {
match signal {
Signal::Value(value) => {
if first.swap(false, Ordering::SeqCst) {
return;
}
let val = (**value).clone();
// Try to push, if full just drop (don't notify)
if buffer.push(val.clone()).is_ok() {
d.notify(Signal::value(val));
}
// If push failed, value is dropped (newest dropped)
}
Signal::Complete => d.notify(Signal::Complete),
Signal::Error(e) => d.notify(Signal::Error(e.clone())),
}
}
});
derived.own(guard);
derived.lock()
}
/// Keep only the latest value - consumer reads at its own pace.
///
/// This is useful when you only care about the most recent value
/// and intermediate values can be skipped. The consumer can call
/// `get()` whenever ready to read the latest.
///
/// Note: This is essentially what a regular Cell already does via
/// `ArcSwap`. This operator is provided for API consistency.
///
/// # Example
///
/// ```
/// use hyphae::{Cell, Mutable, Gettable, BackpressureExt};
///
/// let source = Cell::new(0);
/// let latest = source.sample_latest();
///
/// source.set(1);
/// source.set(2);
/// source.set(3);
/// // Consumer reads when ready - gets 3, skipped 1 and 2
/// assert_eq!(latest.get(), 3);
/// ```
#[track_caller]
fn sample_latest(&self) -> Cell<T, CellImmutable>
where
T: CellValue,
Self: Clone + Send + Sync + 'static,
{
// Since Cell already uses ArcSwap, this is essentially a passthrough
// that creates a derived cell holding the latest value
let derived = Cell::<T, CellMutable>::new(self.get());
let weak = derived.downgrade();
let first = Arc::new(AtomicBool::new(true));
let guard = self.subscribe(move |signal| {
if let Some(d) = weak.upgrade() {
match signal {
Signal::Value(value) => {
if first.swap(false, Ordering::SeqCst) {
return;
}
// Just update to latest, no buffering
d.notify(Signal::Value(value.clone()));
}
Signal::Complete => d.notify(Signal::Complete),
Signal::Error(e) => d.notify(Signal::Error(e.clone())),
}
}
});
derived.own(guard);
derived.lock()
}
}
impl<T, W: Watchable<T>> BackpressureExt<T> for W {}
#[cfg(test)]
mod tests {
use std::sync::atomic::{AtomicU32, Ordering};
use super::*;
use crate::{Gettable, Mutable};
#[test]
fn test_drop_oldest() {
let source = Cell::new(0);
let buffered = source.drop_oldest(3);
let count = Arc::new(AtomicU32::new(0));
let c = count.clone();
let _guard = buffered.subscribe(move |_| {
c.fetch_add(1, Ordering::SeqCst);
});
assert_eq!(count.load(Ordering::SeqCst), 1); // Initial
source.set(1);
source.set(2);
source.set(3);
assert_eq!(count.load(Ordering::SeqCst), 4);
// Buffer full, but drop_oldest still emits (drops from buffer)
source.set(4);
assert_eq!(count.load(Ordering::SeqCst), 5);
}
#[test]
fn test_drop_newest() {
let source = Cell::new(0);
let buffered = source.drop_newest(3);
let count = Arc::new(AtomicU32::new(0));
let c = count.clone();
let _guard = buffered.subscribe(move |_| {
c.fetch_add(1, Ordering::SeqCst);
});
assert_eq!(count.load(Ordering::SeqCst), 1); // Initial
source.set(1);
source.set(2);
source.set(3);
assert_eq!(count.load(Ordering::SeqCst), 4);
// Buffer full - new values dropped, no emission
source.set(4);
assert_eq!(count.load(Ordering::SeqCst), 4); // Still 4
source.set(5);
assert_eq!(count.load(Ordering::SeqCst), 4); // Still 4
}
#[test]
fn test_sample_latest() {
let source = Cell::new(0);
let latest = source.sample_latest();
source.set(1);
source.set(2);
source.set(3);
// Latest value is 3
assert_eq!(latest.get(), 3);
}
#[test]
fn test_drop_oldest_forwards_complete() {
let source = Cell::new(0);
let buffered = source.drop_oldest(3);
let completed = Arc::new(AtomicBool::new(false));
let c = completed.clone();
let _guard = buffered.subscribe(move |signal| {
if let Signal::Complete = signal {
c.store(true, Ordering::SeqCst);
}
});
source.complete();
assert!(completed.load(Ordering::SeqCst));
}
#[test]
fn test_drop_newest_forwards_complete() {
let source = Cell::new(0);
let buffered = source.drop_newest(3);
let completed = Arc::new(AtomicBool::new(false));
let c = completed.clone();
let _guard = buffered.subscribe(move |signal| {
if let Signal::Complete = signal {
c.store(true, Ordering::SeqCst);
}
});
source.complete();
assert!(completed.load(Ordering::SeqCst));
}
}